Fishing is a big deal in Wisconsin. In addition to being part of our heritage, our strong angling tradition makes it a $1.4 billion industry here. We are home to 1,000 miles of Great Lakes shoreline, over 15,000 inland lakes, and nearly 3,000 trout streams running over 13,000 miles throughout the state. One fish that lives in all those types of waters and is particularly prized by anglers is the brook trout.

Known to many simply as “brookies,” brook trout are the only species of trout native to Wisconsin. They are easily recognized by the marble pattern on their backs, and spotted sides that include distinctive red spots with blue halos.

Brook trout live and thrive only in very clean and cold water, and have been noted to prefer spawning in areas where groundwater mixes with surface waters. Beyond just being prized as sport fish, these water quality needs of brook trout also make them very useful scientifically, as an “indicator species.” This means that since they are so sensitive to changes in their environment, studying them can give us an early indication of problems that may occur. Some of the most significant are:

Water acidity Brook trout can’t live in waters that have been acidified, for example, by the acid mine drainage that can come from mine sites or the acid rain that can result from coal power plant emissions.

Water clarity Brook trout rely on their sense of sight to feed, so they have trouble when excessive erosion or runoff (caused, for example, by deforestation or bad riverbank control practices) increases turbidity.

Dissolved oxygen Brook trout need high levels of dissolved oxygen in the water, which can be cut down by excessive biological activity. Since organisms in the water use oxygen both when they grow and when they decompose, brook trout suffer when nutrient pollution like phosphorus causes algal blooms, which drastically reduces dissolved oxygen in the water.

Water temperature Brook trout need very cold water, preferring temperatures less than 66 degrees or so. This is especially important for spawning, when they do best with water between 40 and 50 degrees. Historically, this indicated where deforestation significantly reduced shading levels on a waterway. More recently, however, the biggest impact has been the general warming trend from climate change. The Wisconsin Initiative on Climate Change Impacts predicts that, in a worst-case scenario, brook trout could be completely wiped out in Wisconsin by mid-century due to warming temperatures.